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Abstract

Microresonator-based frequency comb generation at or near visible wavelengths would enable applications in precise optical clocks, frequency metrology, and biomedical imaging. Comb generation in the visible has been limited by strong material dispersion and loss at short wavelengths, and only very narrowband comb generation has reached below 800 nm. We use the second-order optical nonlinearity in an integrated high-Q silicon nitride ring resonator cavity to convert a near-infrared frequency comb into the visible range. We simultaneously demonstrate parametric frequency comb generation in the near-infrared, second-harmonic generation, and sum-frequency generation. We measure 17 comb lines converted to visible wavelengths extending to 765 nm.

(a) The effective mode index dispersion of the fundamental TE mode in near IR spectral range and the third-order TE mode in the visible spectral range. One can see that around the pump wavelength of 1540 nm (red X), phase-matching occurs. (b) The group velocity dispersion (GVD) of the fundamental TE mode shown in (a). The low level of anomalous GVD at the pump wavelength of 1540 nm allows for efficient frequency comb generation. (c) Conversion efficiency vs. wavelength for χ(2) SHG and SFG processes, estimated using Eq. (1). Phase-matching bandwidth for SFG is wider than for SHG, allowing for multiple comb lines to be converted into the visible range.

(a) Near-IR CW pump laser and frequency comb generation. Several different states of the frequency comb are shown. (b) Visible second-harmonic generation and frequency comb lines for corresponding spectra in (a). The vertical scales have been offset for clarity. Comb spacing is preserved by the SFG process for all states of the frequency comb shown. Inset micrograph shows the visible light generated by the device. These wavelengths fall just outside the normal range of the CCD camera, so the red color that is seen by the naked eye is distorted. A microheater was fabricated on this device for thermal tuning but was not used in this experiment.

Seeded SFG experiment. (a) Near-IR pump and signal lasers are coupled into the ring resonator. An additional line is seen symmetric about the pump, resulting from FWM. (b) The visible spectrum shows SHG of the pump wavelength and SFG of pump wavelength mixing with both the signal and FWM idler lines. (c) Conversion efficiency vs. pump power for SFGsignal, showing conversion proportional to the pump power. (d) Conversion efficiency vs. pump power for SFGidler, showing conversion proportional to pump power.